The present invention relates to a microtip emissive cathode electron source and to its production process. It more particularly applies to the production of flat display screens.
French patents 2 593 953 and 2 623 013 disclose display means by cathodoluminescence excited by field emission and which incorporate a microtip emissive cathode electron source.
FIG. 1 diagrammatically shows a known microtip emissive cathode electron source described in detail in French patent 2 623 013. This source has a matrix structure and optionally comprises on an e.g. glass substrate 2, a thin silica film 4. On the latter are formed a plurality of electrodes 5 in the form of parallel conductive strips serving as cathode conductors and constituting the columns of the matrix structure. Each of the cathode conductors is covered by a resistive coating 7, which can be continuous (except at the ends in order to permit the connection of the cathode conductors to the polarizing means 20).
An electrically insulating layer 8, made from silica, covers the resistive coating 7. Above the insulating layer 8 are formed a plurality of electrodes 10, once again in the form of parallel conductive strips. These electrodes 10 are perpendicular to the electrodes 5 and serve as grids, which constitute the rows of the matrix structure.
The known source also has a plurality of elementary electron emitters (microtips), one of which is diagrammatically shown in FIG. 2. In each of the intersection zones of the cathode conductors 5 and the grids 10, the resistive coating 7 corresponding to said zone supports e.g. molybdenum microtips 12 and the grid 10 corresponding to said zone has an opening 14 facing each of the microtips 12. Each of the latter substantially adopts the shape of a cone, whose base rests on the coating 7 and whose apex is level with the corresponding opening 14. Obviously, the insulating layer 8 also has openings 15 permitting the passage of the microtips 12.
For information, the following orders of magnitude are given:
thickness of insulating layer 8: 1 micrometer,
thickness of a grid 10: 0.4 micrometer,
diameter of an opening 14: 1.4 micrometer,
diameter of a base of a microtip 12: 1.1 micrometer,
thickness of a cathode conductor 5: 0.2 micrometer,
thickness of a resistive coating: 0.5 micrometer.
The essential object of the resistive coating 7 is to limit the current in each emitter 12 and consequently homogenize the electron emission. In an application to the excitation of spots (pixels) of a display screen, this makes it possible to eliminate excessively bright dots.
The resistive coating 7 also makes it possible to reduce breakdown risk at the microtips 12 through limiting the current and thus preventing the appearance of short-circuits between rows and columns.
Finally, the resistive coating 7 allows the short-circuiting of a few emitters 12 with a grid 10, the very limited leakage current (a few .mu.A) in the short-circuits does not disturb the operation of the remainder of the cathode conductor. Unfortunately, the problem caused by the appearance of short-circuits between the microtips and a grid is not solved in a satisfactory manner by a device of the type described in French patent 2 623 013.
FIG. 3 diagrammatically shows a microtip. A metal particle 16 causes a short-circuit of the microtip 12 with a grid 10 and in this case all the voltage applied between the grid 10 and the cathode conductor 5 (Vcg approximately 100 V) is transferred to the terminals of the resistive coating 7.
In order to be able to accept a few short-circuits of this type, which are virtually inevitable due to the very large number of microtips, the resistive coating 7 must be able to withstand a voltage close to 100 V, which requires its thickness to exceed 2 .mu.m. In the opposite case, it would lead to a breakdown due to the heat effect and a complete short-circuit would appear between the grid and the cathode conductor making the electron source unusable.